Maximum number of exclusive locks

From: "Daniel Verite" <daniel(at)manitou-mail(dot)org>
To: "pgsql-general" <pgsql-general(at)postgresql(dot)org>
Subject: Maximum number of exclusive locks
Date: 2016-09-13 12:03:28
Message-ID: 07d8bc2e-a501-4f5e-baf8-a7e99e8f4b48@mm
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Hi,

When deleting large objects, an exclusive lock is grabbed on each
object individually. As a result, a transaction that does it en
masse can encounter this error:

ERROR: out of shared memory
HINT: You might need to increase max_locks_per_transaction.

I would expect the maximum number of lo_unlink() in the same
transaction to be capped at:
max_locks_per_transaction * (max_connections + max_prepared_transactions)
per documentation:
https://www.postgresql.org/docs/current/static/runtime-config-locks.html

"The shared lock table tracks locks on max_locks_per_transaction *
(max_connections + max_prepared_transactions) objects (e.g., tables);
hence, no more than this many distinct objects can be locked at any
one time"

But in practice, on an otherwise unused 9.5 instance, I've noticed
that this query:

select count(lo_unlink(oid)) from (select oid
from pg_largeobject_metadata limit :LIMIT) s;

with these settings:

max_locks_per_transaction | 512
max_connections | 30
max_prepared_transactions | 5

starts failing at LIMIT=37133, although I'd expect this to
happen, in the best case, at LIMIT=512*(30+5)=17920.

Nothing to complain about, but why would the above formula
underestimate the number of object locks actually available
to a transaction? Isn't it supposed to be a hard cap for such
locks?

Best regards,
--
Daniel Vérité
PostgreSQL-powered mailer: http://www.manitou-mail.org
Twitter: @DanielVerite

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